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Author

Yu, Le

Date of Issue

2016

School

School of Chemical and Biomedical Engineering

Abstract

Complex metal oxides, especially in the form of nanostructures, have attracted increasing attention as promising electrode materials for electrochemical energy storage (EES) systems such as lithium-ion batteries (LIBs) and hybrid supercapacitors (HSCs) owing to their superior structure features compared to the simple binary metal oxides. In this project, we focus the rational design and synthesis of two representative types of complex metal oxide-based electrodes including hybrid systems and ternary metal oxides, as well as their applications in LIBs and HSCs. The hybrid metal oxides systems include TiO2@Fe2O3 integrated electrodes and NiCo2O4@MnO2 core-shell structures. In the former case, we have designed a smart nanocomposite composed of aligned TiO2 nanotube arrays (TNAs) with attached Fe2O3 hollow nanorods on both the outer and inner surface. The unique core-shell architecture has been created by introducing FeOOH nanospindles onto the TNAs with a follow-up thermal treatment in air. Benefitting from their structural superiorities, such a hierarchical structure displays excellent lithium storage property with a remarkable areal capacity and a good cycle performance. Hierarchical NiCo2O4@MnO2 core-shell heterostructured nanowire (NW) arrays grown on nickel foam are produced via a facile two-step hydrothermal method. The core-shelled NW arrays on nickel foam, own obvious advantages as an electrode for HSCs due to the strong synergistic effect from the mesoporous NiCo2O4 NW core and the sheet-like MnO2 shell. Impressively, such advanced integrated array exhibits a large areal capacitance, satisfying rate and cycling performance in a LiOH solution. The three typical examples of ternary metal oxides systems are CoxMn3-xO4 micro-/nanostructures array, NiCo2O4 hierarchical structures and yolk-shelled Ni-Co mixed oxide prisms. Hierarchical CoxMn3-xO4 micro-/nanostructures array has been developed through a facile two-step process. Co-Mn precursor with tunable morphology and composition on stainless steel matrix is firstly synthesized via tuning the solvent components during the solvothermal process. The mesoporous CoxMn3-xO4 micro-/nanostructures with various macroscopic features from nanowires to nanosheets can be finally obtained through a further annealing process at 600 oC with a slow ramping rate. The distinct micro-/nanostructures endow the CoxMn3-xO4 sample with outstanding rate capacities at high current densities with enhanced capacity retention after 30 cycles as anode material for LIBs. Other than the binder-free array structures, micro-/nanostructured particles can overcome the limit of loading mass for the active materials. Similar to CoxMn3-xO4 system, hierarchical NiCo2O4 structures composed of wire-like or sheet-like mesoporous building blocks can be prepared via a facile two-step route. The outer morphologies can be easily tuned via the control of the reaction solvents. These distinct hierarchical architectures could ensure the fast electronic/ionic dynamics, large surface area, and enhanced mechanical integrity. When applied as electrode materials for high-performance HSCs, these NiCo2O4 micro-/nanostructures deliver large specific capacitance with long calendar life. Starting with prism-like metal acetate hydroxide, Ni-Co mixed oxide nanoprisms with yolk-shelled characteristic and tunable chemical element can be prepared by a simple oxidation process in air. The formation mechanism can be attributed to the fast thermally driven contraction process. Thanks to the intriguing porous structure with desirable composition, these yolk-shelled Ni-Co oxides represent greatly enhanced electrochemical activities as energy source for hybrid supercapacitors with amazing capacitance retention over 15000 cycles. Moreover, these interesting materials can effectively buffer the structural stress during Li ions insertion/extraction process, leading to highly reversible specific capacities at diverse densities.